The invention relates to a method of manufacturing a heavy metal fluoride glass composition in which suitable starting materials are mixed in the presence of a fluoridizing reagent and melted together to form the glass composition.
Heavy metal fluoride glass compositions are fluoride glass compositions in which the network former is not beryllium fluoride BeF.sub.2 but a heavier metal, often zirconium fluoride ZrF.sub.4. Such glass compositions are suitable for the manufacture of infrared transparent components such as windows, glass fibres and lenses, in particular for the wavelength range from 2 to 7 .mu.m. They can also suitably be used as matrix material for lasers and as material in which efficient upconversion processes can be carried out. The glass compositions can be used in the form of both bulk glass and glass fibres. Particular uses lie in the field of optical telecommunications because of the theoretically very small optical losses of the glass compositions.
Heavy metal fluoride glass compositions are generally prepared from anhydrous and oxygen-free components because the presence of oxygen, water, hydroxyl groups and various anionic impurities in the glass composition gives rise to crystallization and the formation of other imperfections. Just like other impurities this causes the light absorption to increase and the stability of the glass composition to decrease. For this reason, such glass compositions are customarily prepared from very pure metal fluorides in an atmosphere containing a fluoridizing reagent. In this manner it is achieved that any impurities present, often oxides and hydroxides, are converted to fluorides and that anionic impurities are replaced by fluorine ions.
In an article by M. M. Broer and R. M. Atkins in J. Mater. Res. 3 (4), pages 755 to 760 (1988), a description is given of a method in which starting materials for the manufacture of heavy metal fluoride glass compositions are pretreated with a fluoridizing reagent such as gaseous NF.sub.3 or SF.sub.6. Said gases are also used during the melting of the starting materials to produce the glass composition. Said article also describes the use of solid NH.sub.4 F.HF as a fluoridizing reagent, which is known per se but which has the drawback of forming water and hydroxyl groups during reaction, which is undesirable.
A disadvantage of the use of very reactive gases such as NF.sub.3 and SF.sub.6 is that the furnace in which the glass composition is melted may be attacked. Reactions with the wall material of the furnace may take place causing the furnace to be subject to substantial wear. Besides, the products formed in such reactions may contaminate the glass composition. In addition, when very reactive gases are used safety measures must be taken.